I love the idea of the UnitsApi module. Unfortunately it isn't the best Drupal citizen. If you look in de issue queue you will see loads of issues asking for support for a specific unit. In my opinion users should be able to provide their own units via the UI.
The plan is to do a rewrite of the module and adding a few submodules. Instead of loading the unit definitions from an XML file each time, it will use entities and allow users to add their own definitions for quantities, units and conversion using the UI. When users indicate that a unit Foo (F) is metric, conversions to and from mF (milliFoo) and kF (KiloFoo) will be done automatically. Conversions between Foo and another unit Bar of the same quantity can be defined by the UI. Such a definitions will create a relation between two unit entities using the relation module. The module should provide full backwards API compatibility with the Units API module.
The list of (sub)modules I'm planning to work on at the moment: